Radiation experts say it is difficult to know the severity of the situation in Japan, as explosions at the country's Fukushima nuclear plant lead to rising levels of radiation around the facility.

Japan's prime minister Naoto Kan has warned radiation levels near the plant, about 250 kilometres north-east of Tokyo, are now harmful to human health.

People living nearby, still reeling from Friday's devastating earthquake and tsunami, have either been evacuated or told to stay in their homes.

Radiation experts say staying indoors is the best way to limit exposure, and that it is useless to wear a face mask.

The International Atomic Energy Agency (IAEA) says radiation levels around the plant are now 400 millisieverts an hour, eight times the amount, every 60 minutes, that nuclear workers are normally supposed to absorb in a year.

Ira Helfand, a medical doctor based in Washington DC who has written extensively on the impacts of radiation, says someone exposed to this level of radiation for a couple of hours would develop radiation sickness.

"This is a very, very large increase from the radiation readings that were recorded just a few hours before the most recent explosion," he said.

He says there are two different types of harm caused by exposure to radiation.

"If you get a high enough dose of total body radiation you'll develop something called radiation sickness and you - over the course of a period of days to weeks - develop nausea, vomiting, suppression of your bone marrow," he said.

"Which allows you to become susceptible to infections, which promotes bleeding and you become weak, dehydrated.

"If you absorb a large enough dose you die from this over a period of several weeks.

"But even if you don't get that kind of large total body dose of radiation, if you inhale or ingest radioactive nuclei like radio-iodine or caesium or strontium or plutonium you can develop cancer subsequently.

"This is a second distinct danger that people will be facing if there is a very large release of radiation in this disaster."

According to the National Academy of Sciences, any exposure to radiation increases a person's risk of cancer.

Peter Burns, who has 40 years experience in radiation safety and was Australia's representative on the United Nation's Scientific Committee on the Effects of Atomic Radiation, says at this stage it appears it is only volatile gases that have been released, which do not pose much of a threat.

"As far as I'm aware the primary containment vessel is still intact and as long as that stays intact then the only materials that are likely to get out will be the volatile ones," he said.

"It's not as dangerous to be immersed in those as the long-term consequences of other materials that might escape if the core itself, as in Chernobyl, was exposed and the fire there just sent clouds of radioactive particles all over eastern Europe."

Mr Burns says people in Japan have been given iodine tablets as a precaution.

"Your thyroid will take up iodine every day, it has a small amount, but if the thyroid's got enough iodine in it, it won't take up any more," he said.

"So basically you give it a big dose of iodine, then for the next few days it won't take up any iodine into the thyroid, so that the radioactive iodine won't go into your blood.

"Once it's in the thyroid it stays there for a few months. Usually the iodine 131 has a half-life of one week so it'll be there for a few weeks."

He says extra clothing or face masks are not the answer to prevent exposure and that the best thing anyone can do is stay indoors.

"The sort of paper face masks are pretty useless for this sort of radiation because it's too volatile, they'll go through," he said.

"If you actually shelter indoors, if there's a cloud passing overhead, and keep all the doors and windows shut then you can reduce the levels in your house."